With the increasing traffic demand, cellular operators plan to evaluate the feasibility of using unlicensed spectrum to improve network capacity in LTE Release-13. The first idea of LTE-LAA is to aggregate the unlicensed carriers as a Supplemental Downlink (SDL) by using the Carrier Aggregation (CA) technology, and a primary carrier thereof is located in licensed bands.
Different from the exclusive use of licensed spectrum, unlicensed spectrum is shared by multiple Radio Access Technologies (RATs) such as Wi-Fi and Radar system, etc. This makes the LAA transmission opportunistic based on the channel conditions and its own traffic demand. Hence one of the distinct features of LTE-LAA is the non-contiguous and opportunistic use of the unlicensed channel. Particularly, in some areas such as in Europe and Japan, the Listen-Before-Talk (LBT) regulation is required to be followed in order to reduce negative impact on nearby coexisting systems. That is, before data transmission, a Clear Channel Assessment (CCA) check will be performed by using “energy detection” for a duration of a CCA period. If the energy level in the channel is below the CCA threshold, the channel may only be occupied for a Channel Occupancy Time between 1 ms and 10 ms. FIG. 1 illustrates the requirements defined by European Telecommunications Standards Institute (ETSI) for frame based equipment, and the basic parameters are given in the following Table 1.
TABLE 1Basic Parameters for frame based equipmentParameterValueCCA period>=20 usChannel Occupancy TimeBetween 1 and 10 msIdle Period>=5% of Occupancy Time
Hence the short term Channel Occupancy Time (between 1 ms and 10 ms) leads to the discontinuous and intermittent transmission on the unlicensed carriers. FIG. 2 illustrates an example of a Hybrid Automatic Repeat Request (HARQ) process on an unlicensed channel such as unlicensed channel #1. It can be seen from FIG. 2 that the base station has to suspend its transmission after 10 ms Channel Occupancy Time to perform the CCA. However, this feature may cause negative impact on HARQ operation on the unlicensed channel as detailed below.
First, this increases HARQ retransmission latency. The longer the unlicensed channel is detected to be busy, the longer the retransmission latency is incurred. Even if the HARQ retransmission packets are finally transmitted successfully, e.g. after tens of milliseconds, the latency for reassembling operation in Radio Link Control (RLC) layer at the User Equipment (UE) side becomes intolerable. In the worst case, these packets even become useless as the RLC layer has already delivered the received RLC Packet Data Units (PDUs) in the order without the missing PDUs to higher layers or has initiates an Automatic Repeat Request (ARQ) retransmission because the RLC reordering timer (for RLC AM/UM mode) is expired. This obviously wastes radio resources.
Further, as per current 3GPP specification, the New Data Indicator (NDI) included in the downlink assignment is used by the UE to determine whether to clear the soft buffer or not. For LTE-LAA, due to the discontinuous feature and lack of the NDI, the UE doesn't know when and whether to clear the soft buffer. If the soft buffer is cleared earlier than the HARQ retransmissions arriving, the HARQ retransmissions become meaningless. On the contrary, if the soft buffer is kept a little longer whereas the unlicensed channel could not be used for retransmission, it becomes inefficient for the buffer management at the UE side.